The determination of permeability and other hydraulic properties of formations surrounding a borehole is very useful in gauging the producibility of the formations, and in obtaining an overall understanding of the structure of the formations. For the reservoir engineer, permeability has generally been considered a fundamental reservoir parameter which has ranked in importance with porosity, fluid saturations, and formation pressure in the description of a reservoir. When obtainable, cores provide important data concerning permeability. However, in situ measurements of permeability (that is, accurate measurements) for different types of formation conditions has been difficult to obtain using existing well-logging techniques. An ideal permeability logging device would perhaps provide a continuous log of horizontal and vertical permeabilities, but no practical device has been proposed which would provide this capability.
Existing techniques have been classified into indirect and direct methods of determining permeability. In indirect methods, permeability is determined from empirical correlations which attempt to express permeability in terms of other measured formation parameters, for example, porosity and saturation. A direct measurement technique involves actual measurement of fluid flow, pressure, etc. and determination of permeability from these measurements. See, for example, U.S. Pat. No. 4,427,944 of Chandler, assigned to the same assignee as the present application, which describes a system for obtaining permeability by measuring streaming potentials.
Existing devices, whose primary use has been for sampling formation fluids, have also been used, with some success, in estimating formation permeability. Formation testing devices which can take repeated samples are disclosed, for example, in the U.S. Pat. Nos. 3,780,575 and 3,952,588. Typically, in this type of device, a hydraulic pump provides pressure for the operation of various hydraulic systems in the device. Sample chambers are provided in the tool to take samples of formation fluid by withdrawing hydraulically operated pistons. Pressure transducers are provided to monitor pressure as the fluid is withdrawn, and pressure can be continuously recorded at the surface. So-called pre-test chambers are also typically provided and are operated to permit more reliable flow during the subsequent fluid withdrawal. Filters can also be typically provided to filter sand and other particulate matter, and pistons can be provided to clean the filters, such as when the tool is retracted.
One type of formation testing device includes an elongated body and a setting arm on setting pistons which are used to controllably urge the body of the device against a side of the borehole wall at a selected depth. The side of the device that is urged against the borehole wall includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted against the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon the fluids can be withdrawn from the formation during pre-test and the actual test.
Existing formation sampling devices have been of limited usefulness in determining formation permeability for a number of reasons. In some instances, attempts have been made to use pressure measurements during fluid withdrawal as an indicator of permeability. If fluid is extracted at a fixed flow rate (independent of permeability), as is typically done, in low permeability formations the pressure drop tends to be too large, and solution gas and/or water vapor forms and can make the results uninterpretable. On the other hand, at high permeabilities, the pressure drop tends to be too small and cannot be accurately measured.
In the U.S. Pat. No. 2,747,401 there is disclosed a method and apparatus for determining hydraulic characteristics, including permeability, fluid pressure, and hydraulic anisotropy, of formations surrounding a borehole. A pressure gradient is obtained in the formations by inserting a probe through the borehole wall. Pressure differences between different points are then used to obtain indications of hydraulic characteristics of the formations. In an embodiment disclosed in the patent, a pair of spaced probes are inserted into the formation, and a pressure gradient is generated by inserting a fluid into the formation at one of the probes (a source probe) at a constant flow rate. The other probe (a measurement probe) is coupled to a pressure responsive device. Pressure is measured at the measurement probe before and after injection of the fluid at the source probe. The permeability of the formation is then obtained using a formula in which permeability is proportional to viscosity times flow rate divided by the change in pressure. The patent points out that the pressure gradient can also be obtained by extracting fluid from the formation and that measurements can be made in more than one direction, for example vertical and horizontal, to obtain indications of both vertical and horizontal hydraulic characteristics.
The type of approach set forth in the U.S. Pat. No. 2,747,401, that is, of establishing a pressure gradient and determining hydraulic characteristics therefrom, is a useful beginning toward obtainment of formation hydraulic characteristics. It can be noted, however, that when measurements are taken over a fixed spacing and over a specific time interval, the extent of the formation that is contributing to the output permeability will be dependent upon the permeability itself (since the permeability is a determining factor in how quickly the pressure pattern spreads out in the formation). Also, the time or times at which pressure measurements are taken is limiting in that longer measurement durations may be more desirable from the standpoint of increasing depth of investigation, but may be less desirable from the standpoint of measured signal strength.
There are a number of approaches which might greatly improve the types and accuracy of determinations of hydraulic characteristics of formations that can be obtained, although they would pose practical difficulties in implementation for a variety of reasons. For example, if a complex model of the formations is assumed, and/or if determinations are to be made at a large number of different times, attempted solutions for hydraulic properties can tend to be overly complex and time consuming, which limits their practicality.
It is among the objects of the present invention to provide improved techniques and apparatus for determining hydraulic parameters of formations with improved accuracy and depth of investigation over a relatively wide range of formation permeabilities. It is also among the objects of the present invention to provide improved techniques and apparatus for determination of formation hydraulic parameters quickly and without undue processing.